(a) Field of the Invention
The present invention relates to a non-magnetic monocomponent color toner composition and a method for preparing the same, and more particularly to a non-magnetic monocomponent color toner composition having a narrow charge distribution, good charging characteristics, good environmental independence, superior image characteristics, high transfer efficiency, and long-term stability caused by significantly improved charge maintenance capability, and a method for preparing the same.
(b) Description of the Related Art
The recent hard-copying and printing techniques using image formation methods, such as electrophotographs, are rapidly moving toward full color from black and white. In particular, the color printer market is expanding very rapidly. In general, formation of color images by full color electrophotography is carried out with three colors comprising cyan, magenta, and yellow, or four colors further comprising black, to present all colors. In this rapidly growing full color market, high image quality, good reliability, compactness, lightweightness, low price, high speed, low energy consumption and recyclability, and so forth are highly required. Improvement and development of image formation methods and toners to satisfy these requirements are widely progressing.
In general, image formation in electrophotography comprises:
1. a charging step of uniformly charging a drum surface;
2. an exposure step of exposing the drum surface and forming an electrostatic latent image;
3. a developing step of developing the latent image on the drum surface using a toner formed on the surface of a developing roller and obtaining a toner image;
4. a transfer step of transferring the toner image;
5. a fixing step of settling the toner image; and
6. a cleaning step of removing toner remaining on the drum surface from the transfer step.
Each step of the image formation process in electrophotography requires the following characteristics from a toner. The developing step requires an appropriate charging of the toner, charge maintenance, and environmental independence. The transfer step requires good transfer characteristics. The fixing step requires low-temperature settlement characteristics and offset resistance. And lastly, the cleaning step requires good cleaning characteristics and contamination resistance. Recently, the above characteristics have become more important with the trend toward high resolution, high speed, and full color.
With regard to long-term maintenance of image quality for repeated printing, there is a method of mixing four colors directly in a photoconductive drum in the transfer step. And recently, indirect transfer image formation has been mainly used in full color printers because it can offer high speed and good image quality. In indirect transfer image formation, a toner image on the drum surface is repeatedly transferred to an intermediate transfer belt by each color, and then the image is transferred paper as a whole.
However, indirect transfer image formation requires more toner transfer steps. Therefore, better and more exact transfer characteristics are required to obtain a good image quality. Also, research on additives, toner shape, surface structure control, and so forth are required to improve charging stability or transfer efficiency, in order to obtain stable long-term and high-quality full color images.
With regard to the cleaning step, reduction of remaining toners after transfer and reducing the cleaner size are important tasks for improving environmental independence. In particular, for a three-color comprising cyan, magenta, and yellow, or a four-color toner further comprising black, toners remaining after transfer are a significant problem.
To overcome these problems of the transfer step and the cleaning step, it is important to reduce remaining toners. For this purpose, it is important to improve transfer efficiency of the toner, and to maintain it. To improve transfer efficiency of the toner, it is necessary to reduce the toner's adhesivity to the photoconductive drum.
Fine particles, such as silica, may be added to the toner to reduce its adhesivity to the photoconductive drum. The fine particles reduce the toner's adhesivity to the drum and improve its transfer efficiency. To obtain good transfer efficiency, many fine particles should be coated on the toner surface. Consequently, the addition amount of the fine particles increases and the toner charging characteristics become poor. Moreover, the fine particles may adhere to electrostatic latent image carriers, and filming or fixing problems may occur. Especially, silica particles may cause problems of image density irregularity at low temperature and humidity, and non-image area contamination at high temperature and humidity, because they are highly environment-dependent.
As a method for improving environmental independence of a toner, addition of inorganic fine particles having electric resistance lower than that of silica particles and good-charge exchange ability, such as titanium oxide particles, is known. However, if inorganic fine particles having lower electric resistance are used, charge distribution of the toner may change easily. Also, poor second transfer when using an intermediate transfer belt or retransfer of wrong sign color toner during multiple transfers may be caused.
A method of increasing resistance of inorganic fine particles by treating the surface with a silane coupling agent, etc. was proposed to solve this problem. However, cohesion of the fine particles becomes so severe that their dispersibility on the toner surface decreases. Also, fluidity of the toner may decrease or blocking may occur due to free cohesioned particles.
Accordingly, research on a color toner having a narrow charge distribution, good charging characteristics and environmental independence, and superior image characteristics, transfer efficiency, and long-term stability, is highly needed.